Mutant Microbes Test Space Radiation Resistance

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Early Earth lacked an ozone layer to act as a shield against
high-energy solar radiation, but microbes flourished by adapting
to or finding other forms of protection from the higher
ultraviolet radiation levels. Now researchers have begun testing
modern microbes to see if they could act as pioneers in the harsh
conditions of extraterrestrial space and other planetary
environments.

One such study from last year looked at Bacillus subtilis, a
common soil bacterium which has become a model
organism for experiments and holds the record for space
survival after spending six years on NASA's Long Duration
Exposure Facility spacecraft.

The ordinary microbe proved capable of evolving a resistance to
UV radiation of up to 3 times higher than that of the original
ancestor or a non-UV-exposed group, after 700 generations lived
and died in an Earth-based lab experiment.

By comparing the radiation-resistant mutants with their ancestor
and the non-UV-exposed group, researchers could almost be certain
that their adaptation to UV did not come from a UV-resistant
specimen hiding among the original bacteria population.

"The significance is that a single organism is actively capable
of reacting and adapting to changes in its environment," said
Marko Wassmann, a radiation biology researcher at the German
Aerospace Center's Institute of Aerospace Medicine in Germany.
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That
adaptation to radiation hints at how some microbes might have
survived a journey to Earth aboard ancient asteroids, according
to the theory
known as Panspermia. Similarly, the adaptive ability
indicates how Earth microbes might be able to colonize harsher
extraterrestrial environments such as Mars, although even the
most radiation-resistant bacteria would face other challenges if
they tried to survive beyond Earth.

The experiment also showed that B. subtilis was capable of
adapting to UV levels even higher than those found on a
primordial Earth – a harbinger of untapped potential that still
lies within some organisms.

Survivability doesn't mean that the bacteria don't have limits.
The researchers estimated that the bacteria undergoing " totally
unshielded exposure " would only survive for two and a half
minutes on the surface of early Earth.

Luckily for B. subtilis, its soil home should provide some
shielding protection under more "naturalistic circumstances" on
Earth. The fact that the bacterium spends most of its life in a
protective spore state also adds to its timeline for survival.

At the same time, the researchers made sure that the apparent
radiation resistance in their experiment did not come from
any kind of shielding. They confirmed the lack of shielding
by looking at the DNA photoproducts molecules that resulted from
UV exposure in both the UV-evolved and ancestor cells to make
sure they were the same.

The bacterial cell ability to repair DNA damage may change in
terms of speed, but the overall repair ability remains unaltered,
according to Wassmann. He and his colleagues continue to analyze
any changes in the repair pathways.

The mutant microbes have not remained idle since their adaptive
changes. Researchers sent the hardy Earth bacteria up to the
International Space Station for a real test of radiation
resistance in the space experiment ADAPT, which was mounted on
the EXPOSE-E platform of the space station's Columbus module. The
samples returned to Earth at the end of September 2009.

The dormant bacterial spores have been exposed to both space and
simulated Martian conditions for 18 months. That meant they had
to survive not only the increased radiation dosage, but also
exposure to vacuum and the loss of water.

"The hypothesis to be tested experimentally in ADAPT was whether
longer-lasting selective pressure by UV radiation results in a
higher UV resistance, as well as in a higher resistance against
further ‘extreme’ environmental factors that exist in space,"
Wassmann said.

The evaluation of the space experiment results has been ongoing
since early 2010. But the Earth-based experiment that created the
mutant microbes has already pointed to the greater considerations
for both the past and future of humanity in space.

"If this single organism is capable of founding a new and
well-adapted population, it has much more potential for
inhabiting new biotopes," Wassmann said. "And this has large
impact on different fields including, but not limited to, the
theory of Panspermia and planetary protection issues."

Given the ability of even small invertebrates
known as water bears to repair DNA after radiation exposure,
more surprises could yet await researchers who study the
bacteria.

The study was detailed in the July/August 2010 issue of the
journal Astrobiology. The researchers at the German Aerospace
Center now have additional B. subtilis samples to examine from
the EXPOSE-R platform that returned to Earth aboard the space
shuttle Discovery on March 9, 2011.